The first time a programmer who was trained in the classical procedural/object oriented history is confronted with the concept of making everything immutable, the first question that comes to mind is, “won’t that make my application slow?” This is because of how most programmers have been trained. Making everything immutable generally means that we must copy a lot of memory from one place to another. Moving memory around is generally considered slow. And so, most programmers dismiss the whole idea as crazy talk. But is it really all that crazy?